Device and method for indicating the expansion of plaster of paris



J. D. WIGGIN 1,746,691

DEVICE AND METHOD FOR INDICATING THE EXPANSION OF PLASTER OF PARIS Feb. 11, 1930.

Filed July 26. 1922 Snowntoz W 1 fwd/e? Patented Feb. .11, 1930 unrrso STATES PATENT OFFICE JOHN D. WIGGIN, F MON'I'CLAIR, NEW JERSEY, ASSIGNOR 130 H. B. WIGGINS SONS COMPANY, OF BLOOMFIELD, NEVV JERSEY, A CORPORATION OF NEW JERSEY DEVICE AND METHOD FOR INDIGATING THE EXPANSION OF PLASTER OF PARIS Application filed July 26,

This invention relates to a new and useful device and method for indicating the expansion or contraction of plaster of Paris and other substance having similar characteristics.

The invention has for an object the provision of a simple device which will illustrate in a comparative way the relative expansions or contractions of various grades of plaster of Paris and other similar materials possessing the properties of expanding and contractmg. In making various castings of plaster of Paris, such as dental plates, which have to be fitted exactly to the shape of the original object, it has been found that the expansion or contraction of the plaster of Paris when set ting causes changes in the form of the casting so that it no longer remains true to the original model but rather presents an inaccurate reproduction thereof. In the making of dental plates, for example, this causes serious diilic-ulties since the final plates must fit into their places exactly. In making various kinds of plaster of Paris compositions for use in this class of work, it has been found desirable to provide some simple device which can readily show the comparative expansions or contractions of variou kinds of plaster of Paris and other materials.

The present invention provides for such a device. The accompanying drawings illustrate an embodiment thereof, and in said drawings,

Figure 1 shows a convenient device for retaining the plastic material against the mold,

Fig. 2 shows the molding member;

Fig. 3 shows the plaster of Paris cast;

Fig. 4 shows the molding members with the plaster of Paris just after it has been inserted and before hardening; and

Fig. 5 shows the same parts after expansion of the plaster of Paris.

The device consists of a molding member 2 having a curved portion 3 preferably in the term of a circular or elliptical arch, the circle being considered a special form of an ellipse; the lower heel portion at at the end of the curve 3 may be curved as shown in the drawing or may have any other suitable shape. The retaining member 1, which has a base 1922. Serial No. 577,571.

and upstanding, preferably parallel, arms,

and the molding member may be placed together so as to produce a mold in which the plaster of Paris cast 5 may be formed. This is allowed to harden sufficiently for removal from the mold, and it is then removed and allowed to expand or contract unconfined in order to assume the form shown in Fig. 5. When the cast is replaced after expansion. has taken place, there will be contact at points 6 only, while an air space or gap will be formed between theplaster of Paris cast and the mold 2 in the space denoted by 7, and

also in the space denoted by 8. The expansion of the plaster of Paris so alters the curved portion of the plaster of Paris cast that it no longer fits into the curved mold exactly, but makes contact at the point 6' only as aforesaid. Should the plaster of Paris or other material contract, contact will be made at point 8, but point 7 will still show an air gap.

The total area of these air gaps and their location is more or less in proportion to the expansion or contraction of the plaster of Paris and therefore readily indicates the quality of the latter. In this way various kinds of plaster of Paris compositions may be quickly compared as to their relative expansibility or contractibility. This forms a ready means for inst-ant examination of any brand of plaster of Paris or other material with a standard product, and accordingly, dentists or other persons using plaster of Paris may readily examine a submitted sample for its desirability in making casts.

The lower portions or abutments 8 of the curve 3 are shown curved but they may advantageously assume other shapes. Ihe retaining member 1 is desirable to assist in molding the plaster of Paris or other material but it may of course be replaced by any other device functioning in the same-manner.

As many apparently widely different embodiments of this invention may be made without departing from the spirit thereof, it is to be understood that I do not intend to limit myself to the specific embodiment thereof except as ated in the appended claims.

I claim:

1. A method of indicating the expansion or contraction of plaster of Paris and similar substances, which comprises molding it against a curved molding surface, removing the surface before the substance has set, allowing it to set and change its dimensions,

and then noting the degree of change in comparison with said molding surface.

2. An instrument for indicating the expansion or contraction of plaster of Paris or similar substances comprising a plaster retaining device having a base and upstanding arms and a movable molding member to fit between and engage said arms and presenting a curved molding surface.

3. -An instrument for indicating the expansion or contraction of plaster of Paris or similar substances comprising a plaster retaining device having a base and upstanding arallel arms, and a molding member fitted tween and engaging said arms and presenting a curved molding surface extending transversely of said arms.

4. An instrument for indicating the expansion or contraction of plaster of Paris or similar substances comprising a plaster retaining device having a base and upstanding I parallel arms, and a molding member fitted etween and engaging said arms and presenting a curved molding surface substantially in the form of an elliptical arch extending transversely of said arms and having heels or ends curving in the opposite direction and extending to the adjacent edges of the arms of the molding member.

In testimony that I claim the foregoing, I have hereunto set my hand this 12th day of June, 1922.

JOHN D. WIGGIN. 

